The statements in this section merely provide background information and may not constitute prior art.
Velocity measurements using a Laser Doppler technique do not inherently resolve the direction of the velocity. The current methods to determine the sign of the velocity tend to be complex, bulky and/or require significant electrical power. Additionally, previously developed systems and methods usually impose a secondary peak at zero velocity. This tends to create a dead band in velocity around zero velocity. For applications where the velocities are low, the dead band in velocity around zero velocity can be a significant performance limitation of such systems.
There are generally two classes of direction resolution solutions. The first class measures both the signal and quadrature of the electrical beat frequency. A Fourier transform of a time series including both signal and quadrature is generated that has both positive and negative frequencies. The second class of solutions imposes a fixed, offset frequency on a local oscillator of the system. The difference between a detected beat frequency and the fixed offset frequency can be both positive and negative, thus resolving the direction ambiguity. Two common methods are used for imposing a frequency offset. An Acousto-Optic Modulator can be used to impose an offset frequency in the range of about 30-250 MHz. A rotating radial grating can be used to impose lower frequency offsets.
The first class of solutions described above (i.e., measuring both signal and quadrature) also requires a complex optical subsystem which cannot be implemented in an optical fiber. With the second solution mentioned above, an acoustic optic-modulator is required. An acoustic optic-modulator is a relatively large and heavy component and typically requires several watts of radio frequency (RF) power to operate. It is also somewhat electrically noisy. The use of a rotating radial grating also has limitations associated with it. For one, this component requires a precision motor to rotate the grating at a fixed frequency. Without extreme care, the rotating grating is susceptible to vibration. Moreover, the use of a rotating radial grating is not easily adapted to an optical fiber system.
In certain Doppler velocimeter applications it may be preferable to avoid the use of optical fiber cabling when implementing the system. However, the above limitations can still exist with regard to being able to resolve the velocity magnitude and sign of the velocity, especially when the magnitude of the velocity is small.